Many large, multi-domain proteins must undergo dramatic structural movements to perform their functions. X-ray crystallography provides static pictures of the protein but does not describe its movement. To more fully understand the function of these proteins, it is useful to study the conformational changes undergone by the protein and the rates of those changes. Fluorescence resonance energy transfer between fluorophore- labeled domains reports on the distances and distance change between those domains. The conformational changes of the enzyme cobalamin-dependent methionine synthase (MetH) can be studyied by labeling its 5 domains with different fluorophores to measure the interactions between the different domains. MetH is important in the pathway for the utilization of folate derivatives and homocysteine. During catalysis, MetH must undergo a complex molecular "dance" to allow each domain to interact with vitamin B12. MetH is an ideal candidate for the study of conformational changes because rates of conformational change can be compared to the previously reported kinetics, and it is known that the chemistry of the reaction is not rate- limiting. This research will provide insight for the study of additional multi-domain proteins and enzymes.